I. Field of the Invention
Aspects of the invention relate to wireless communications. More particularly, the aspects of the invention relate to estimating reverse link loading in a wireless communication system.
II. Description of the Related Art
If a minimum acceptable signal quality is specified, an upper bound on the number of simultaneous users, which can communicate through a base station, can be calculated. This upper bound is commonly referred to as the pole capacity of a system. The ratio of the actual number of users to the pole capacity is defined as the loading of the system. As the number of actual users approaches the pole capacity, loading approaches unity. A loading close to unity implies potentially unstable behavior of the system. Unstable behavior can lead to degraded performance in terms of voice quality, high error rates, failed handoffs, and dropped calls. In addition, as loading approaches unity, the size of the coverage area of the base station shrinks so that users on the outer edge of the no-load coverage area are no longer able to transmit sufficient power to communicate with the base station at an acceptable signal quality.
For these reasons, it is advantageous to limit the number of users that access the system so that loading does not exceed a specified percentage of the pole capacity. One way to limit the loading of the system is to deny access to the system once the loading of the system has reached a predetermined level. For example, if the loading increases above 70% of the pole capacity, it is advantageous to deny requests for additional connection originations, and to refrain from accepting hand-off of existing connections.
In order to limit the loading on the reverse link to a specified level, it is necessary to measure the reverse link loading. Reverse link loading of a base station is not solely a function of the number of remote units that are operating within the coverage area of the base station. Reverse link loading is also a function of interference from other sources. The front-end noise of the base station itself is a significant source of interference. In addition, other remote units operating on the same frequency within the coverage area of nearby base stations may contribute significant interference.
One means by which the reverse link loading can be measured is by averaging the measured signal to interference operation point of all active connections within the coverage area. This approach has several drawbacks. The signal to interference operation statistics of the active connections provide an indication of system performance. However, they do not provide any information concerning the amount of interference from remote units located in the coverage area of other base stations. In addition, when a remote unit is in soft hand-off between two or more base stations, it is likely that the actual signal to interference ratio at which the reverse link signal is received at any one base station is significantly beneath the signal to interference ratio set point determined by the system, thus, falsely indicating an extremely high loading level. For these reasons, measuring the average signal to interference operation point of all active connections within a base station does not provide an accurate measure of reverse link loading.
A second and simple means of determining reverse link loading is to simply count the number of active users in the base station. However, because the level of interference from other sources significantly affects loading, it should be clear that the number of users is not necessarily a good indication of reverse link loading. In addition, the effects of soft hand-off greatly decrease the correlation between the number of active users and the actual loading at the base station.
A third means of estimating the reverse link loading is to attempt to derive the reverse link loading based upon an estimate of the forward link loading. However, in a typical system the forward and reverse links do not operate at the same frequencies. Consequently, the interference from the coverage areas of adjacent base stations can be different on the forward link than on the reverse link. In addition, the effects of fading are independent as between the forward and reverse links. Furthermore, loading is a function of a data rate of a particular user. Therefore, the forward link performance is not perfectly correlated with reverse link performance.
If one of these inaccurate methods of estimating the reverse link loading is used, the system cannot accurately determine whether connection blockage is necessary. If calls are blocked unnecessarily, the capacity of the system is unnecessarily decreased. On the other hand, if the loading is permitted to approach the pole capacity, the probability of dropping a significant number of active connections increases. For this reason, it is important to have an accurate estimation of the reverse link loading.
In his book entitled “CDMA: Principles of Spread Spectrum Communication” (Addison-Wesley Wireless Communications, 1995), Dr. Andrew J. Viterbi defines reverse link loading as a function of the total received power perceived at the base station receiver. The reverse link loading X is directly related to the total power received by the base station according to the following formula:
                                          P            a                                P            n                          =                  1                      1            -            X                                              (        1        )                            where: Pa is the actual power received at the base station;                    Pn is the power received at no external loading (e.g. the power due to the thermal noise floor of the base station); and            X is the reverse link loading in terms of the ratio of actual loading to pole capacity.Or equivalently, expressed in terms of X, Equation 1 takes on the following expression:                        
                    X        =                                            P              a                        -                          P              n                                            P            a                                              (        2        )            For example, this formula states that at 50% loading (X=0.5), the total power received at the base station is twice that which is received at no loading.
Given the relationship shown in Equation 1, current base station loading X can be determined based upon a known no load power level and an actual measurement of the total power received at the base station. Note that the actual power measurement should be filtered with an appropriate time constant in view of the time constant at which the power control operation varies the transmit power of the remote unit. In addition, if the reverse link operates at variable data rates resulting in gated transmissions from the remote units, the actual power measurement should be filtered to average the effects of the gated transmissions on the instantaneous power measurement.
The dynamic range of the relative power measurement (Pa/Pn) is not large in a typical system. For example, as the loading X increases from 0 to 90% of the pole capacity, the ratio of (Pa/Pn) increases from 0 to 10 decibels (dB). Typically, base station loading X is limited to about 60-75% of the pole capacity. As X increases from 0.6 to 0.75, the ratio of (Pa/Pn) increases from about 4 to about 6 dB. Therefore, to accurately limit the loading of the reverse link, the ratio of (Pa/Pn) should be measured with less than 1 dB of error in order to avoid over- or under-estimation of the loading.
While this approach appears to be straightforward, in reality, it is difficult to achieve consistently required accuracy of the relative power measurements. For example, accurately measuring the noise floor (e.g., Pn) of a base station in an operating environment is difficult. In addition, even if an accurate measurement of the noise floor could be made at one time, the noise floor is sensitive to gain and noise figure variations due to temperature, aging and other phenomena, and, hence, the noise floor power level changes as a function of time. Without a means of accurate measurement, any admission control algorithm based upon Equation 2 will likely block connections when no blocking is necessary or admit connections resulting in potentially unsteady system behavior.
In addition to the no load power measurement, the actual power received at the base station must also be measured. The measurement of the absolute power level using power meters or automatic gain control circuits is extremely difficult within an accuracy of a few dB. In order to achieve this sort of accuracy in an absolute power measurement, the cost and size of the measurement apparatus becomes prohibitive.
In another improved method for determining cell loading, a system enters a period of silence. During the period of silence, a remote test unit generates a reverse link signal. A base station demodulates the reverse link signal and generates a series of closed loop power control commands for the remote unit. The remote unit responds to the power control commands by adjusting the level at which it transmits the reverse link signal. As the system operating point changes in response to the new operating conditions, the series of commands are accumulated to determine a transmit gain adjustment value corresponding to the period of silence, TGA(0). Once normal system operation is resumed, the base station demodulates the reverse link signal from the remote test unit and generates a series of power control commands for the remote unit. As the system operating point changes in response to the normal operating conditions once again, the series of power control commands are accumulated to determine a transmit gain adjustment value for the current system loading, TGA (t). Using TGA(0) and TGA(t), the system loading is determined. This method for determining cell loading is described in detail in copending U.S. patent application Ser. No. 09/204,616, entitled “METHOD AND APPARATUS FOR LOADING ESTIMATION”, assigned to the assignee of the present invention and incorporated by reference herein.
The use of code division multiple access (CDMA) modulation techniques is one of several techniques for facilitating communications in which a large number of system users are present. Other multiple access communication system techniques, such as time division multiple access (TDMA) and frequency division multiple access (FDMA) are known in the art. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein. The use of CDMA techniques in a multiple access communication system is further disclosed in U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING SIGNAL WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, assigned to the assignee of the present invention, of which the disclosure thereof is incorporated by reference herein.
There has been an increasing demand for wireless communications systems to be able to transmit digital information at high rates. One method for sending high rate digital data from a remote station to a central base station is to allow the remote station to send the data using spread spectrum techniques of CDMA. One method that is proposed is to allow the remote station to transmit its information using a small set of orthogonal channels, this method is described in detail in U.S. Pat. No. 6,396,804, entitled “HIGH DATA RATE CDMA WIRELESS COMMUNICATION SYSTEM”, issued May 28, 2002, assigned to the assignee of the present invention and incorporated by reference herein.